The shear rate (sometimes called the strain rate or velocity gradient), which is the proper measure of the rate of deformation in the fluid undergoing shear flow. In shear flow-where we imagine the flow as hypothetical layers of fluid flowing over each other-we define the relevant parameters as (see Figure 1) σ the shear stress (force per unit area) at the boundary of the fluid to produce the flow, and Quantitatively, viscosity is defined as the stress in a particular ideal flow-field divided by the rate of deformation of the flow. Viscosity can depend on the type of flow (shear and/or extensional), its duration and rate, as well as the prevailing temperature and pressure. It is worth emphasizing that the above expressions are not fundamental laws of nature, but rather definitions of viscosity.Viscosity is that property of a fluid which is the measure of its resistance to flow (i.e. In the Couette flow, a fluid is trapped between two infinitely large plates, one fixed and one in parallel motion at constant speed u can be important is the calculation of energy loss in sound and shock waves, described by Stokes' law of sound attenuation, since these phenomena involve rapid expansions and compressions. Although it applies to general flows, it is easy to visualize and define in a simple shearing flow, such as a planar Couette flow. Viscosity is the material property which relates the viscous stresses in a material to the rate of change of a deformation (the strain rate). For instance, in a fluid such as water the stresses which arise from shearing the fluid do not depend on the distance the fluid has been sheared rather, they depend on how quickly the shearing occurs. In other materials, stresses are present which can be attributed to the rate of change of the deformation over time. Stresses which can be attributed to the deformation of a material from some rest state are called elastic stresses. For instance, if the material were a simple spring, the answer would be given by Hooke's law, which says that the force experienced by a spring is proportional to the distance displaced from equilibrium.
In materials science and engineering, one is often interested in understanding the forces or stresses involved in the deformation of a material. In a general parallel flow, the shear stress is proportional to the gradient of the velocity. A fluid with a high viscosity, such as pitch, may appear to be a solid. Otherwise, the second law of thermodynamics requires all fluids to have positive viscosity such fluids are technically said to be viscous or viscid. Zero viscosity is observed only at very low temperatures in superfluids.
So for a tube with a constant rate of flow, the strength of the compensating force is proportional to the fluid's viscosity.Ī fluid that has no resistance to shear stress is known as an ideal or inviscid fluid. This is because a force is required to overcome the friction between the layers of the fluid which are in relative motion. In such a case, experiments show that some stress (such as a pressure difference between the two ends of the tube) is needed to sustain the flow through the tube. For instance, when a viscous fluid is forced through a tube, it flows more quickly near the tube's axis than near its walls. Viscosity can be conceptualized as quantifying the internal frictional force that arises between adjacent layers of fluid that are in relative motion. For liquids, it corresponds to the informal concept of "thickness": for example, syrup has a higher viscosity than water. The viscosity of a fluid is a measure of its resistance to deformation at a given rate.
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